The spinal cord has typically been difficult to treat medically because it is not easily accessible and its vital function in the body demands that techniques used for medical treatment disturb the function of the cord minimally or not at all. However, medical treatment of the spinal cord may be necessary for a number of reasons, such as to alleviate pain, to control certain aspects of the nervous system, to replace deteriorating structure or to treat disease. Several techniques have been developed to treat the spinal cord, depending on the type of treatment required.
For example, electrical stimulation of a spinal cord, is one well-known technique in the art. Such electrical stimulation in the spinal cord has been shown to be effective in relieving certain types of pain. Selective nerve stimulation has also been used to control various aspects of the nervous system for such objects as controlling urination, fecal incontinence or penile erection.
Techniques are also known in the field of reconstructive surgery to replace the deteriorating bone structure of the vertebral canal with prosthetic structures, such as a prosthetic lamina. In one surgical procedure, called a laminectomy, the tissue is spread away from around the cord and the ligaments between two bony vertebral elements are cut. Then all or part of one section of the lamina is removed and replaced by a prosthetic apparatus. The prosthetic is secured to the remaining bone and provides additional support.
Furthermore, some techniques, such as that provided in U.S. Pat. No. 4,313,438 to Greatbatch and U.S. Pat. No. 5,330,477 to Crook, both of which are incorporated herein by reference, show that electrical stimulation of bone tissue can accelerate the growth of the tissue, create a germicidal environment for curing tissue and bone infections and may inhibit the growth of tumors.
Delivery of drugs to the spinal cord for such purposes as creating a germicidal environment or relieving pain is also desirable. For example, in one technique, a small hole is drilled in the bony cylinder of the spinal cord and a catheter is placed into the hole through the bone and through the dura mater towards the side of the cord that is affected by pain. Pain-relieving drugs may then be delivered through the catheter. Spasticity-relieving drugs may be similarly delivered.
Successful or beneficial electrical stimulation and drug delivery to the spinal cord remain difficult to achieve because of the cord's location and function.
A lead may be implanted adjacent the spinal cord to provide stimulation, such as described in U.S. Pat. Nos. 4,285,347; 5,121,754; 5,501,703; 5,628,317; and 5,643,330; all of which are incorporated herein by reference in their respective entireties. It may be difficult to implant such leads in the proper location. Over time, particularly in the first two or three weeks following lead implantation, the position of the lead with respect to the spinal cord may change inadvertently and in an undesirable manner. Because of such displacement, the lead may not provide stimulation to the precise target spinal area where it was originally implanted. Such positioning changes may impair the clinical benefit of the stimulation provided by the lead.
Another technique is disclosed in U.S. Pat. No. 5,484,445 to Knuth, incorporated herein by reference in its entirety, where stimulation electrodes are anchored directly to the bone. Problems such as tearing of the bone tissue may occur when the electrodes or leads are directly fixed to the bone. The techniques of Greatbatch and Crook described above also require that electrical components be anchored directly to the remaining bone tissue.
Similar difficulties are faced in delivering drugs to or near the spinal cord. A chronically implanted catheter may change position as the patient in which it is implanted moves or may not stay in place because the patient in whom it is implanted has poor posture. However, if a catheter is anchored to the bone, there exists the danger of damaging bone tissue. Finally, drilling a hole to insert a catheter or to create an opening for drug delivery may lead to bone damage, particularly if the bone is already in the process of deteriorating.
In the case of many patients requiring spinal cord treatment, the bony elements of the cord are in such a deteriorated state that it is detrimental to anchor any element to the bone. Because most medical treatment of the spinal cord is intensive and may last over several weeks or even years, the patient may be forced to lie still while electrical stimulation is performed or while drugs are dispensed through an implanted catheter.
Other disclosures relating to prosthetic vertebral elements include the U.S. Patents listed below in Table 1.
U.S. Pat. No. Title 5,562,736 Method for Surgical Implantation of a Prosthetic Spinal Disc Nucleus 5,147,404 Vertebra Prosthesis 4,554,914 Prosthetic Vertebral Body 4,401,112 Spinal Fixator 4,369,769 Spinal Fixation Device and Method 4,141,365 Epidural lead electrode and insertion needle 3,978,499 Surgical Implant and Method for its Production
As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, at least some of the devices and methods disclosed in the patents of Table 1 and elsewhere above may be modified advantageously by using the teachings of the present invention.
It would be desirable therefore to provide a structure that can remain appropriately placed for effective delivery of medical treatment to a spinal cord over a reasonable period of time, even if the patient is mobile. It would also be desirable to provide a structure to which components for effective medical treatment may be anchored without causing further damage to a spinal cord. It would also be desirable to provide a structure capable of delivering a variety of treatments to the spinal cord, depending on the needs of the individual patient.